There is a class of instrument that measures the coordinates of a point by sending a laser beam to a retroreflector target in contact with a point on an object. The instrument determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest.
The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. Another instrument known as total stations or tachymeters may, in some cases, measure a retroreflector. A broad definition of laser tracker, which encompasses total stations, is used throughout this application.
Ordinarily the laser tracker sends a laser beam to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The vertex, which is the common point of intersection of the three mirrors, is located at the center of the sphere. Because of this placement of the cube corner within the sphere, the perpendicular distance from the vertex to any surface on which the SMR rests remains constant, even as the SMR is rotated. Consequently, the laser tracker can measure the 3D coordinates of a surface by following the position of an SMR as it is moved over the surface. Stating this another way, the laser tracker needs to measure only three degrees of freedom (one radial distance and two angles) to fully characterize the 3D coordinates of a surface.
For a laser tracker to measure the distance and two angles to a retroreflector target, the laser beam from the tracker must be able to reach the retroreflector without encountering any obstructions that block the beam. In practice, it is sometimes necessary or convenient to measure a location on an object that is not in the line of sight of the laser beam from the tracker. Such measurements are sometimes referred to as “hidden point” measurements.
Two different methods have been devised for measuring hidden points. The first method is to use a device called a retroprobe. A retroprobe uses a mirror in combination with a retroreflector to create a virtual image of the retroreflector at the location of a physical probe tip. By placing the probe tip at the obstructed (hidden) point and the mirror's reflection of the retroreflector within the line of sight of the laser tracker, the coordinates of the hidden point can be measured. A disadvantage of the retroprobe is that, in some instances, the retroprobe cannot be oriented in the required geometry.
A second method for measuring a hidden point is to use a tracker and probe that together have the ability to measure six degrees of freedom. The six degrees of freedom (six-DOF) include the three degrees of freedom mentioned earlier—distance and two angles—and in addition three angles of probe orientation—for example, pitch, roll, and yaw. By attaching a stylus with a probe tip to the probe, it is possible to measure the coordinates of a hidden point. A disadvantage of such six-DOF laser trackers is that they are more expensive than laser trackers that measure just one distance and two angles.
What is needed is an apparatus or method that enables a laser tracker to measure hidden points.